First of all, the hard cap of +4 is indeed under debate. Parts of the code were written with the assumption it applies, but it's not phrased clearly enough. If you follow the restriction, it indeed applies to foci and control-rigs normally. Note that I'm saying normally.
Second, the restriction is supposed to apply to NON-attribute parts of the tests. So you have both the Augmented Maximum of the attribute, and the max bonus on the test-pools. There's also some things that don't count against the limit. See the SRM guide for some info on how it's done in Missions.
In other words, you roughly have:
(Attribute + max augmented bonus of 4) + (Skill + max bonus of +4) + Specialization/Expertise
OR
(Attribute + max augmented bonus of 4) + (Attribute + max augmented bonus of 4) +

(let's assume a max bonus of 4 here for consistency, even if it's not in the rules).
On a defense test that's 5(9)+5(9)+3, the limit would likely be intended to be max +4, so 22 dice max.
As a sidenote: Say you have a Force 8 Spellcasting Focus. You use Spellshaping at a -4 penalty. That puts you at (+8-4=4, so follows the max bonus of 4). But yes, with Background Count nerfed down, the +4 bonus limit is the only real nerf to Foci. It does help balance things out a bit, so I would suggest to consider it.